A reluctance motor is an electric motor in which torque is produced by the tendency of its moveable part to move to a position where the inductance of the excited winding is maximized. The motion may be rotary or linear, and the rotor may be interior or exterior. The "winding" usually consists of a number of electrically separate circuits or phases. In a motoring operation, each phase is usually excited when its inductance is increasing, and is unexcited when its inductance is decreasing. The opposite is true for a generating operation. The reluctance motor is a fully controlled variable speed drive system, when powered from a suitable power converter circuit. It is capable of producing high torque at low speeds, thus making it particularly well suited for use in traction and other industrial applications. The reluctance motor is also useful in applications requiring a motor that must operate at high speeds.
A switched reluctance (SR) motor is a doubly salient motor. In this regard, both the stator and the rotor have salient poles. A coil, wound around each stator pole is connected with the coil on the diametrically opposite stator pole to form a phase winding. The reluctance of the flux path between the two diametrically opposite stator poles varies as a pair of rotor poles move in and out of alignment. Because inductance is inversely proportional to reluctance, the inductance of the phase is a maximum when the rotor is in the aligned position, and a minimum in the non-aligned position. A pulse of positive torque (i.e., motoring torque) is produced if current flows in a phase winding as the inductance of that winding is increasing. A negative torque (i.e., generating torque) contribution is avoided if the current is reduced to zero before the inductance starts to decrease again.
A controller is desired which can increase and decrease the current in a phase winding as fast as possible. However, it has been observed that abrupt decreases in the current in a phase winding at "turn off," causes mechanical oscillations in the motor, which makes for noisy motor operation. Moreover, abrupt changes in the current (i.e., "hard" inductive switching), leads to switching losses in the associated switching device (e.g., commutation transistor), which may result in lower efficiency and higher operating temperatures. The higher operating temperatures lead to a shortened life for the switching device. In addition, there are significant harmonics generated in response to the sudden change in current. Accordingly, there is a need for a controller which can rapidly, but less abruptly, decrease the current in a phase winding.